Television shading control circuit



Feb. 23, 1943. o. iii. SCHADE,

TELEVISION SHADING CONTROL C`IRC'UKIT Filed sept. so, 19211 Prefconodnoe Ver , A, INVENTO y goo hun. rr

' ATTORNEY EEEEVHSHUN SKNG 60ML 'E' Otto H. Schade, est ilaidrvell, N.3., assigner to Radio @orporation or of Deiaware .or erica, acorporation Appiication September 3i), mail, Serial No. 412,933

il. Claims. (Si. lim-7.2)

This invention relates to shading control for television transmittingsystems and, more particularly, to circuits whereby the backgroundintensity or average light value of various selectable portions of thetelevision image may be controlled in order to compensate for inherentoperating characteristics of the system.

- In present day electronic transmitting stations a televisiontransmitting tube is used in which a light responsive electrode (forexample, of the mosaic type) is provided, upon which a charge image isproduced in accordance with the light rvalues of an optical imageprojected thereon.

The light responsive or mosaic electrode is then scanned in a systematicorder by a cathode ray beam which is deilected in bilateral directionsin order that electric signals or potential .variations may be producedwhich are representative of the charge values which yare present on themosaic electrode. The operation of such a transmitting tube,particularly the scanning of the mosaic and the resulting production ofpicture signals, depends to a considerable extent upon secondaryelectron emission .from the mosaic electrode surfaceand the averagevalue of the signals produced by the transmitting tube are to a certainextent controlled by the secondary electron emissive characteristics ofthel mosaic electrode. In the production of television picture. signals,due to the scanning of the mosaic electrode, a certain relatively highfrequency signal series is produced which is indicative of the lightvalues or detail of the optical image projected on the mosaic.

However, due to various reasons, the average value of the high frequencysignals may shift or change considerabhr during the scanning of -thecomplete mosaic area. or, in fact, during `the scanning of each line ofthe area, that is, during each deection of the cathode ray beam in aparticular direction. Such changes in the average value of the signalseries may actually exceed the amplitude of the high frequency voltagevariations thereof. Such variations in the average value of the picturesignal series, .as derived from the signal plate of the televisiontransmitting tube, are represented in the received picture by animproper and undesirable background levei of thepicture in localizedareas or portions of the entire picture image, often together with aloss of detail in such areas. This eiect is known as dark'spo Oneportion of the picture may api pear White, and, in fact, so light as tobe devoid of substantially all picture detail, whereas another portionof the picture may, under extreme conditions, appear almost black. Thechange in the average value ofthe picture signals, as. produced by thetelev' 'ion transmitting tube, may be caused by severa differentfactors, one of which is the lack of #uniformity of the secondaryelectron emissive effect throughout the entire light responsiveelectrode surface. Another cause of dark spot is the possible unequaldistribution of the produced secondary electrons over the scanned sideof the light responsive 'electrode of the mosaic. Various other causesand effects may also contribute to the existence of dark spot in thepresently used television transmitting systems.

The dark spot voltage variations, as stated above, may be the result ofseveral different effects, but it has been noticed that the dark spotvoltage variation, or wave form, remains relatively constant for anyparticular transmitting tube under favorable operating conditions and isnot altered in any excessive amount when difierent light images areprojected on. the mosaic. There is, however, a slight variation in thedark spot of a particular television transmitting tube when diiierentlight images are projected on the mosaic, but the percentage'change inthe dark spot voltage variation isrelatively small and is of such avalue as would ordinarily be noticed only vwhen considerable change ispresent in the average value of the light projected on thelightresponsive electrode.

in order to compensate for the dark spot, various systems have beendevised by means of which there is inserted in the picture signal seriesa compensating voltage of relatively low frequency as compared with the'frequency of the picture signals, the wave shape of the darkspotcorrectlng voltage being controlled both as to amplitude and wave form.Such previously used dark spot correction circuits have heretoforenormally employed signals of sine wave formation or harmonies of a sinewave, as well asother related wave forms, the amplitude and thefrequencies of which have been controlled in accordance with the Vwaveform of the particulardark spot signal variation which is to becompensated. Examples of previously used dark spot correction systemsare to be found in the patent to Bedford, No. 2,166,712, issued July 18,1939, and in the applica-s tion of Seeley, Serial No. 281,359, led June27 1939 (RCA D-l'Zl) The present invention, therefore, has for one ofits purposes the provision of a dark spot correcting or shading circuitwhich is simple in operation and which involves a minimum of electricalapparatus and controls.

Another purpose of the .present invention resides in the provision of adark spot correction and shading control circuit in which voltagevariations of substantially sawtooth wave form are used and in whichcomponents of such a voltage may be derived -i'rom the circuit.

Another purpose of the present invention resides inthe vprovision tlf ashading control circuit in which the shading of'the image may be con'-trolled both in a horizontal and in a vertical direction selectively andindependently.

A still further purpose of the present invention `resides in theprovision of a dark spot correction or shading control circuit in whichthe wave form of the shadingimpulses, as applied to the televisionsystem, may the particular correction required.

Stilianother purpose of the present invention resides in the provisionof a dark spot correction or shading control circuit in which the systemand the means used for correction in a horizontal direction aresubstantially identical with the system and means used for correction ina vertical direction, and the entire circuit is of such a nature thatthe individual shading signals may be combined into a composite signalseries which may be superimposed upon the picture signals as developedby the transmitting tube, in order that the resultant signal variationsmay correspond to the actual light variations of the optical irnage,which is projected on the light responsive electrode of the televisiontransmitting tube.

A further purpose of the present invention re sides in the Iprovision ofa dark vspot correction or shading control circuit for developingsignals which may be applied directly to the signals derived directlyfrom the pre-amplifie1 associated with the television transmitting tubein order that such dark spot correction may be exercised prior to anyappreciable amplification of the -picture signals to thereby eliminateanyl attenuation of excessive amplification of portions of the picturesignals as a result ofv the non-linear operation characteristics ofvarious of the television signal amplifying tubes, when such tubes areop- I is shown, which may be a conventional Iconoscope, including vanelectron gun structure i2 for generating a focused` cathode ray beam anda mosaic electrode i4, with which is associated y a signal plate. -Thecathode ray beam is deflected in mutually perpendicularl directions byelectromagnetic and/ or electrostatic means in order that the cathoderay beam may scan the mosaic electrode. When an optical light image isprojected on the mosaic electrode, certain charge conditions will resultand when the mosaic electrode is scanned by the cathode ray beam,picture signals are produced in the. output load circuit iny cluded inthe pre-amplifier i6. The picture signais which are available at thispoint in the'system are, as explained above, not truly representative ofthe light values. but contain certain inbe altered in accordance with.be controlled by the same voltage variations as are used to control thedeflection of the cathode `ray beam.

-Figure 1 shows the vertical deflection power tube I8, which includes acathode, a control electrode, andean anode, and the controllingelectrode ofthe vertical deflection power tube I8 is supplied withenergy or voltage variations of vertical deflection frequency. Thesevoltage variations are condenser 22. sistance of tube i8 includes afixed resistance 24 I .and an anode. The tube by Way of a condenser 40.

applied 'tothe' terminals 20 and are impressed upon the Acontrolelectrode of tube I8 by way of The control electrode or grid reand apotentiometer 26, with which cooperates a movable contact 28. Inasmuchas voltage variations of the vertical deflection frequency are appliedto the control electrode of tube i8, thesel same voltage variations maybe derived from the movable contact 28 of the potentiometer 2l, and theintensity of the voltage variations available at that point is, ofcourse, a function of the position of the movable contact 28 along itscooperating potentiometer resistance 28.

A vertical shading tube 30 is provided, which includes at least acathode, a control electrode.

30 is preferably a pentode of the sharp cutoii? type. Thevoltagevariations, as derived from the potentiometer contact 28, are applied tothe control electrode of tube 30 by way of condenser 32. Agridvresistance 34 is connected to the control electrode of e tube 30and the control electrode is supplied with a variable negative potentialin order to control the operating characteristic of the regionofoperation of the tube 30. In order to provide an adjustable negativepotential for application to the control electrode of tube 30, a sourceof negative potential is connected to the terminal 36. A decouplingresistance 38 is connected to the terminal 3B at one end and the otherend of the decoupling resistance is connected to ground A potentiometer42 and a fixed resistance 43 are connected in series and extend from thecoupling resistance 38 to ground or a point of fixed potential. Thecontrol electrode resistance 34 of the tube 301s connected to themovable contact 44 of the potentiometer 42. By adjusting the position ofthe contact 44 along the potentiometer 42 varying degrees of negativepotential may be applied to the control electrode of tube .80 in orderthat the potential bias of the control electrodeY with respect to itscathode may be controlled. The anode circuit of tube 30 includes a loadresistance 46 which is vconnected between the anode and the terminal 48,to which a source of positive potentialis normally connected.

When a system such as that so far described has been provided, voltagevariations of vertical deflection frequency may be applied to thecontrol electrode of tube 30 in varying degrees of intensity, asdetermined by the position of the contact 28 on the potentiometer 26.The potentiometer 26 is separated from the fixed resistance 24 in orderto provide more accurate control of the intensity of the signals whichare to be applied to tube 38.l The wave form, as well as the intensityof the voltage variations appearing at the anode of tube 80, is afunction not only o! the intensity of the applied voltages but also ofthe grid bias voltage which is applied to the tube.

by means of an adjustment of the potentiometer L contact 44, the voltagevariations having lan intensity as represented by the curve 54 willcause 'voltage variations similar to' those represented by the curve 58to appear at the anode of. tube 88. If, however, the bias which 'isappliedto' tube 38 is set at a value corresponding to the dotted line58, then voltage variations having a wave form similar to those shown bythe curve 68 will cause voltage variations of relatively low amplitude,such as indicated by the curve 62, to appear in the anode circuit. Ifthe applied voltage variations are increased in intensity, as indicatedby lthe curve 64, and the negative bias on the tube 88 is increased to avalue as represented bythe dotted line 66, then the voltage variationsappearing at the anode of tube 38 will correspond to the curve shown at88. Accordingly, it may be seen thatthe intensity of thevoltage'variations appearing atthe anode of tube 30 may be controlledand, furthermore, the length of, time duration of the voltage variationsmay be readily controlled by alterlng the bias applied tov tube 30. Inother words, by increasing the bias applied to tube 38, and bysimultaneously increasing the amplitude of the voltage variationsapplied to the control electrode of tube 30, a continuous change in theshading wave form may be accomplished. Furthermore, the time durationthat the voltage variations at the anode of tube 80 are presentmay alsobe controlled so that the shading will be conned to a greater or lesseramount of a vertical deflection of the scanning cathode ray beam. lThevoltage variationswhichl are produced at the anode of tube 38 are thenlapplied to the output` circuit of the television transmitting tube I8 byway of condenser 10 in order that they may be mixed with the producedpicture signals. The composite wave form, ln-

' cluding picture signals and vertical shading cone trol signals, isthen available at the output electrodes 12.

' The horizontal correction circuit is somewhat similar to the abovedescribed vertical correction provided with a grldresistance including axed resistance 18 and potentiometer resistance 88. Cooperating with.potentiometer 80 is movable contact 82 by means of which voltagevariations of horizontal deflection frequency may be derived in varyingdegrees of intensity.

A horizontal shading control tube 84 is provided which includes atleasta cathode, a control electrode, and an anode. This tube, as in lo thecase of thevertical shading tube, is prefercircuit, and, m Figure 1, thehorizontal deection power tube 18 is shown. This power tube normally isin theform of a beam power tube, such as the type 6116, sinceconsiderable energy is necessary to deilect the cathode ray beam inthehorizontal direction at the desired rate. It is to be understood thatthe energy from the anodes of tube I8 and tube 13 isapplied to thedeflecting coils and their deecting plates of the television',

transmitting tube, although, for reasons of sim-l plicity, suchconnection is not shown in Figure 1.1" The control electrode of. tube 18is supplied with voltagev variations of horizontal deection frequency,these voltage variations being applied to terminals 14. The voltagevariations are applied to the denserk 16.

control electrode through lthe cn- The -control electrode of tube 18 isthe position of the movable contact 94 alongthe potentiometer' 98,'varying degrees of negative biasing potential may be applied to thecontrol electrode of tube 84. The anode of the horizontal shading tube84 is connected to a source of positive potential by means of the loadresistance 46 which is common to both the vertical shading tube 30 'andthe horizontal shading tube 84.

With the horizontal shading control circuit, as so far described,voltage variations of horizontal deiiection frequency and of variableintensity and duration may becaused to appear at the anode of tube k84by' adjusting potentiometer contacts 82 and 94. variations, as arecaused to appear at the anode of tube 84, are applied to the televisionItransmitting tube circuit `through condenser lil along with thevertical shading control voltage variations from tube 38. l'

There are instances wheny it is desirable to use voltage variationsother than of substan- `tially sawtooth wave form, since frequently thedark spot variation does not vary linearly. Aecordingly, .a condenser 98and a potentiometer 99 are provided and are connected between ground (orva point of fixed potential) and a conductor for supplying voltagevariations to the control electrodeof the horizontal shading tube 84. Byadjusting the movable contact |08 along the potentiometer 99, thecondenser 98 may be effectively applied or removed from the circuit invarying degrees in order to attenuate the high frequency component ofthe voltage variations of sawtooth wave form. When the high ,frequency.components of the voltage variations are suppressed, then the wave formtakes the form of a distorted sawtooth wave form or even a parabola andthe voltage variations of distorted or parabolic wave form are thenapplied to the control electrode of the horizontal shadlngtube 84. i

It may be seen, therefore, that in addition to being able to control theintensity and the time duration of the voltage variations,"as appearingat the anode of tube 84,l the wave form of these voltage variations mayalso be controlled.

Although in Figure 1 no' circuit is shownin thel vertical dark spotcorrection channel -i'or' o altering the wave form, a parabola or waveform control' circuit may be incorporated between potentiometer 26 andtube 30, if it is desired.

Through the use of the present invention, as above described, it ispossible to compensate for These voltage pletely overcome the eiect's.of the undesired dark spot voltage variation. By an adjustment of thefew controls provided, proper intensity balance and dark spotlcorrection may be accomplished. Furthermore, it will be observed thatthe system described herein is relatively simple and, in fact, includesonly two tubes, namely, the vertical shading tube30 and the horizontalshading tube 85, in addition to the usual tube components in thetelevision transmitter. When the various adjustment are properly set, aproperly shaded video or picture signal output may be derived vfromthe-output terminals 12.

Although the present invention is described as" being applied to atelevision transmitter of the type wherein a single sided mosaicelectrode M is used, it is obvious that the system may readily beapplied to any type of television transmitting tube where dark spotcorrection is necessary. A system such as shown and described above may,of course, be readily applied to the socalled image-dissector tube orthe system may be applied to al transmitting tube of the imageintensifying type. Furthermore, it is not necessary that the correctionsignal for compensat- -ing for dark spot be applied directly to theoutput of the pre-amplifier of the television transmitting tube, asindicated in Figure 1, although it is preferable to apply the correctionsignals at this point, since the correction signals may readily beapplied to any portion of the complete transmitting system.

Although the system shown and described herein is rather specific innature, it is to be understood that various alterations and modi-.iications may be made therein without departing from the spirit andscope thereof, and it is desired that any and all such modifications beconsidered within the purview of the present 2,312,054 vthe dark spotsignal and vto substantially co ode of said tube whereby the tube willoperate on different selected portions of its operation characteristiccurve to distort the wave form and to cause the voltage variationsappearing at the anode of said tube to be confined to selected p0rtionsof each scanning cycle.

A 3. A shading control circuit for a television transmitting systemincluding a discharge tube having a cathode, a control electrode and ananode, means for applying voltage variations of substantially sawtoothwave form between the cathode and vcontrol electrode of said tube. thevoltage variations corresponding in frequency to one of the scanningrates. means for varying the amplitude. of the applied voltagevariations. means including a load circuit for maintaining the anodepositive with respect to the cathode, means for manually varying thepotential bias between the control electrode and the cathode of saidtube whereby the tube will operate on different selected portions ofitscharacteristic curve to distort thefwave form and tocause the voltagevariations appearing at th-e anode of said tube to be confined toselected portions of each scanning cycle, and whereby the amplitude ofthe distorted wave form may be controlled.

invention, except as limited by the hereinafter l appended claims.

WhatIclaimis:

`1. A shading control circuit for a television transmitting systemincluding a discharge tube having'a cathode, a control electrode and ananode. means for applying Vvoltage variations of 'a predetermined waveform between the cathode and control electrodeof said tube, the voltagevariations corresponding in frequency to one of the television scanningrates, means for controlling the amplitude of the applied voltagevariations, means i'or maintaining the anode positive with respect tothe cathode, and means for varying the potential bias between thecontrol electrode and the cathode of said tube whereby the tube willoperate on different portions of its characteristic curve to distort theapplied wave form and to cause the voltage variations appearing at theoutput of said tube to be connedto variable portions of each scanningcycle.

2. A shading control circuit for a television transmitting systemincluding a discharge tube having a. cathode, a control electrode and ananode, means for applying voltage variations o! substantially sawtoothwave form between vthe cathode and control electrode of said tube, the

voltage variations corresponding in frequency to one of the scanningrates, means for controlling the amplitude of the applied voltagevariations, means including a load circuit for maintaining the anodepositive with respect to the cathode. and means for manually varying thepotential bias between the control electrode and the cath- 4. AV shadingcontrol circuit for a television transmitting system including adischarge tube having a cathode, a control electrode and an anto thecathode, and means for altering the average potential bias between thecontrol electrode and cathode to operate the tube at different portionsof its characteristic curve to distort the applied wave form and tocause the voltage variations appearing at the output of the tube to becon- L trollable in amplitude and to be confined to selectable portionsof 'each scanning cycle.

;5. A shading control circuit for a television transmitting systemincluding a discharge tube having a cathode, a control electrode and ananode, means for applying cyclically varying voltages of substantiallysawtooth wave form and of a `'frequency corresponding to one of thetelevision scanning rates between the cathode and control electrode ofsaid tube, means for varying the amplitudeof the applied voltagevariations. means including a load circuit for maintaining the anodepositive with respect to the .cathode, and means for manually alteringthe potential bias between the control electrode and cathode whereby thetube will operate on different portions of its characteristic curveto'distort the applied wave form and to cause the voltage variationsappearing at the anode of said tube tolbe controllable in amplitude andto be confined-to selectable portions of each scanning cycle. A

6. A shading control circuit for a television transmitting systemincluding a discharge tube having a cathode, a control electrode and anan'- ode, means for applying cyclically varying voltages o!substantially sawtooth wave form and oi' a frequency corresponding toone of the television scanning rates between the cathode and control ofa predetermined wave form between the cathode and the control electrodeof said tube. the frequency of the voltage variations corresponding toone of the television scanning rates, means connected between thecathode and the control electrode of said discharge. tube to vary theattenuation of the high frequency component of the voltage variations,means for maintaining the anode positive with respect to the cathode,and

means for varying the bias applied between control electrode and cathodewhereby the tube may be caused to operate on-a selected portion of itscharacteristic curve to thereby distort the wave form of the appliedvoltage variations and to cause the voltage variations appearing at theoutput electrode of the tube to be confined to selectable portions ofeach scanning cycle.

8. A shading control circuit for a television transmitting systemincluding a discharge tube having a cathode, a control electrode and anput electrode of said tube to be distortedv and to be confined to avariable selected portion of each scanning cycle.

10. A shading control circuit for a television transmitting systemincluding a discharge tube having a cathode, a control electrode and ananode, .a source of voltage variations of substantially sawtooth waveform and of a frequency corresponding to one of the television scanningrates, manually controlled means for varying the attenuation of the highfrequency component of the voltage variations, means for applyingvoltage variations between the control electrode and the cathode of saiddischarge tube, means for varying the amplitude of the applied voltagevariations, means including a load circuit for mainanode, means forapplying the volta-ge variations of substantiallysawtooth wave formbetween the cathodeand the control electrode oi said tube. the frequencyof the voltage variations corresponding to one of the televisionscanning rates, means for controlling the amplitude of the appliedvoltage variations, manually operable means connected betweenthe'cathode and the control electrode of said discharge tube to varylthe attenuation of the high frequency Vcomponent age variationsappearing at the anode of the tube to be conned to selectable portionsof each scanning cycle.

9. A shading control circuit for a television transmitting systemincluding a discharge tube having a cathode, a control electrode and ananode', a source of voltage variations of a predetermined wave form andof a frequency correspending to one of the television scanning rates,means for varying the attenuation of the high frequency component of thevoltage variations, means for applyinsthe voltage variations between thecontrol electrode and the cathode of said discharge tube, means forvarying the amplitude of taining the anode positive with respect to thecathode and manually controlled means for varying the bias voltageapplied between control electrode and cathode whereby the tube may becaused'to operate a selected portion of its characteristic curve tocause the voltage variations appearing at the anode of said tube to bedistorted and to be confined to a variable selected portion of eachscanningcycle.

11. A shading control circuit for a television transmitting systemincluding a rst electron discharge tube having a cathode, a controlelectrode )and an anode, means for applying cyclicallyvarying voltagevariations of substantially saw tooth wave form and of a frequencycorresponding to one of the television scanning rates between .thecathode and control electrode of said tube, a

second electron discharge tube having a cathode, l

a control electrode and an anode,/means for applying voltage variationsof a predetermined wave form between the cathodel and the controlelectrode of said second electron discharge tube,v

the frequency of the voltage variations corresponding to another of thetfevision scanning tween the control electrode and cathode of the twotubes whereby they may be caused to operate on independent selectedportions of their characteristic curves to thereby distort the waveforms of the voltage variations and to cause the voltage variationsappearing at the anodes of the tubes to be conned to selectable portionsof each .scanning cycle of each of the scanning rates.

OTIO H. SCHADE.

